Doctor blades are commonly used for affecting creping of paper in such paper making machines and for other uses in paper-making machines. In that the doctor blade is normally in contacting relation with the surface of a rotating cylinder, the tip of the doctor blade is subject to wear. As this wear progresses, the doctor blade's effectiveness in forming the creped paper product diminishes. That is, progressive wear of the doctor blade may induce progressive diminution of a particularly important property of the product being made or the material being processed by the apparatus in which the doctor blade is disposed.
For example, doctor blades used for creping paper on a tissue paper making machine precipitate progressively greater loss of machine direction tensile strength of the paper as doctor blade wear progresses. This is particularly true in installations where the impact angle progressively changes as wear occurs due to the way the doctor blade is mounted. Commonly, in such machines, creping blades are replaced with new or newly sharpened blades after a product property of particular importance has been reduced to a predetermined minimum acceptable level by doctor blade wear, or after other observed deterioration of the normal doctor blade performance is apparent.
Processes of forming paper towel and tissue products using conventional wet pressed technology utilize such a doctor blade. In this case, the sheet is folded back upon itself thereby breaking many intrafiber bonds with the creping process imparting considerable softness to the fibrous sheet. However, such a creping process suffers from two fundamental drawbacks stemming from the wear inherent in the creping process. The first being that noted above wherein the blade must be replaced at rather frequent intervals as blade wear typically renders conventional blades unusable after a few hours of use. Further, during the operation of the creping process between blade changes, the softness of the creped product immediately after a blade change can vary considerably from the softness of the product made immediately prior to a blade change as blade wear during the intervening time causes the creping geometry to vary leading to both differing degrees of breakage of the intrafiber bonds and to differing configurations of refolding of the sheet back upon itself.
In one effort to reduce the rate of progression of diminution of important product properties due to doctor blade wear is set forth in U.S. Pat. No. 4,919,756. Therein, doctor blade wear is reduced by continually adjusting the impact angle of the doctor blade. That is, as set forth therein, a method and apparatus for continually adjusting the impact angle of the doctor blade to at least partially offset negative ramifications of progressive doctor blade wear are set forth. For example, as noted hereinabove, in paper-making machines for making creped tissue paper, a negative effect of progressive doctor blade wear is progressive diminution of machine direction tensile strength of the paper. That is, machine direction tensile strength of the paper is inversely related to doctor blade wear, which wear is directly related to operating time. This progressive lessening of the paper's machine direction tensile strength can be at least partially offset or compensated for by adjusting the impact angle of the doctor blade. Accordingly, this approach varies the angle and pressure of the contact of the doctor blade with the rotating drum of the paper-making apparatus while utilizing conventional doctor blades.
Doctor blades, when in a operation, may be either fixed or reciprocally mounted in the creping apparatus. Such blades function satisfactorily for a varying period of time, the duration of which depends upon the material from which the blade is formed, the condition of the roll surface, the speed at which the roll is revolved as well as other factors. Once the blade is worn, it becomes necessary to interrupt production and replace the blade with a new or resharpened blade thereby providing a new creping edge. This interruption while unavoidable, is manifestly economically undesirable and is particularly serious in the case of creping blades, which presently exhibit a short life span. Considerable effort has been dedicated to overcoming the aforementioned problems. In some cases, so called continuous creped blades are used, while in other cases creping is eliminated from the tissue manufacturing process all together by using a rush transfer process. In such a process, the web is transported by a first moving support and transferred to a subsequent support which is moving a slower speed. Both of these processes introduce considerable complexities into the system which offset many of the advantages and by such processes.
Clearly, there is a need for a creping blade which may be utilized in present conventional creping processes wherein the life of the blade itself is considerably extended thereby minimizing the interruptions in the creping process due to the changing of the blades regardless of the particular application of the blade. Further, there is a need for a creping blade which exhibits an extended operational life as compared to conventional creping blades such that a more uniform softness of the creped product is experienced due to the extended life and minimized blade changes.